Thymus-derived (T) lymphocytes in the mouse have been found to play a major role in the production of acute inflammation and myofiber necrosis in Coxsackievirus B-3 infected hearts. An in vitro model which will permit the direct observation of the capacity of immune lymphocytes to exert cytotoxic reactions against viral infected mouse myocardium has been developed. The characteristics of this in vitro cytotoxic reaction are being studied.